A resonance Raman spectroscopic and CASSCF investigation of the Franck–Condon region structural dynamics and conical intersections of thiophene

Exploring the ultrafast and isomer-dependent photodissociation of iodothiophenes via site-selective ionization

C-I bond extension and fission following ultraviolet (UV, 262 nm) photoexcitation of 2- and 3-iodothiophene is studied using ultrafast time-resolved extreme ultraviolet (XUV) ionization in conjunction with velocity map ion imaging. The photoexcited molecules and eventual I atom products are probed by site-selective ionization at the I 4d edge using intense XUV pulses, which induce multiple charges initially localized to the iodine atom. At C-I separations below the critical distance for charge transfer (CT), charge can redistribute around the molecule leading to Coulomb explosion and charged fragments with high kinetic energy. At greater C-I separations, beyond the critical distance, CT is no longer possible and the measured kinetic energies of the charged iodine atoms report on the neutral dissociation process. The time and momentum resolved measurements allow determination of the timescales and the respective product momentum and kinetic energy distributions for both isomers, which are interpreted in terms of rival 'direct' and 'indirect' dissociation pathways. The measurements are compared with a classical over the barrier model, which reveals that the onset of the indirect dissociation process is delayed by ∼1 ps relative to the direct process. The kinetics of the two processes show no discernible difference between the two parent isomers, but the branching between the direct and indirect dissociation channels and the respective product momentum distributions show isomer dependencies. The greater relative yield of indirect dissociation products from 262 nm photolysis of 3-iodothiophene (cf. 2-iodothiophene) is attributed to the different partial cross-sections for (ring-centred) π∗ ← π and (C-I bond localized) σ∗ ← (n/π) excitation in the respective parent isomers.

From skin sensitizers to wastewater: the unknown photo-deactivation process of low-lying excited states of isothiazolinones. A non-adiabatic dynamics investigation

Isothiazolinones represent a class of heterocyclic compounds widely used in various applications, including as biocides in cosmetics, detergents, and paints, as well as in industrial wastewater treatment. Indeed, the presence of isothiazolinones in the environment and their associated potential health hazards have raised significant concerns. In this study, a non-adiabatic dynamics investigation was conducted using state-of-the-art methodologies to explore the photochemistry of isothiazolinones. A simplified model, isothiazol-3(2H)-one (ISO), was employed to represent this compound class. The study validated the model and demonstrated that ISO can return to its ground state through the cleavage of the S-N or S-C bonds, with no significant energy barrier observed. Non-adiabatic dynamics simulations provided insights into the time scales and detailed processes of isothiazolinone photodissociation. The preferred route for deactivation was found to be the cleavage of the S-N bond. This research enhances our understanding of the photodeactivation processes of isothiazolinones and their potential environmental impact.

A Local Diabatisation Method for Two-State Adiabatic Conical Intersections

A methodology to locally characterize conical intersections (CIs) between two adiabatic electronic states for which no nonadiabatic coupling (NAC) vectors are available is presented. Based on the Hessian and gradient at the CI, the branching space coordinates are identified. The potential energy surface around the CI in the branching space is expressed in the diabatic representation, from which the NAC vectors can be calculated in a wave-function-free, energy-based approach. To demonstrate the universality of the developed methodology, the minimum-energy CI (MECI) between the first (S1) and second (S2) singlet excited states of formamide is investigated at the state-averaged complete active space self-consistent field (SA-CASSCF) and extended multistate complete active space second-order perturbation theory (XMS-CASPT2) levels of theory. In addition, the asymmetrical MECI between the ground state (S0) and S1 of cyclopropanone is evaluated using SA-CASSCF, as well as (ME)CIs between the S1 and S2 states of benzene using SA-CASSCF and time-dependent density functional theory (TDDFT). Finally, a CI between the S1 and S2 excited states of thiophene was analyzed using TDDFT.

Probing C-I bond fission in the UV photochemistry of 2-iodothiophene with core-to-valence transient absorption spectroscopy

The UV photochemistry of small heteroaromatic molecules serves as a testbed for understanding fundamental photo-induced chemical transformations in moderately complex compounds, including isomerization, ring-opening, and molecular dissociation. Here, a combined experimental-theoretical study of 268 nm UV light-induced dynamics in 2-iodothiophene (C4H3IS) is performed. The dynamics are experimentally monitored with a femtosecond extreme ultraviolet (XUV) probe that measures iodine N-edge 4d core-to-valence transitions. Experiments are complemented by density functional theory calculations of both the pump-pulse induced valence excitations and the XUV probe-induced core-to-valence transitions. Possible intramolecular relaxation dynamics are investigated by ab initio molecular dynamics simulations. Gradual absorption changes up to ∼0.5 to 1 ps after excitation are observed for both the parent molecular species and emerging iodine fragments, with the latter appearing with a characteristic rise time of 160 ± 30 fs. Comparison of spectral intensities and energies with the calculations identifies an iodine dissociation pathway initiated by a predominant π → π* excitation. In contrast, initial excitation to a nearby n⟂ → σ* state appears unlikely based on a significantly smaller oscillator strength and the absence of any corresponding XUV absorption signatures. Excitation to the π → π* state is followed by contraction of the C-I bond, enabling a nonadiabatic transition to a dissociative π→σC-I* state. For the subsequent fragmentation, a relatively narrow bond-length region along the C-I stretch coordinate between 230 and 280 pm is identified, where the transition between the parent molecule and the thienyl radical + iodine atom products becomes prominent in the XUV spectrum due to rapid localization of two singly occupied molecular orbitals on the two fragments.

Combined first-principles statistical mechanics approach to sulfur structure in organic cathode hosts for polymer based lithium-sulfur (Li-S) batteries

Polymer-based batteries that utilize organic electrode materials are considered viable candidates to overcome the common drawbacks of lithium-sulfur (Li-S) batteries. A promising cathode can be developed using a conductive, flexible, and free-standing polymer, poly(4-thiophen-3-yl)benzenethiol) (PTBT), as the sulfur host material. By a vulcanization process, sulfur is embedded into this polymer. Here, we present a combination of electronic structure theory and statistical mechanics to characterize the structure of the initial state of the charged cathode on an atomic level. We perform a stability analysis of differently sulfurized TBT dimers as the basic polymer unit calculated within density-functional theory (DFT) and combine this with a statistical binding model for the binding probability distributions of the vulcanization process. From this, we deduce sulfur chain length ("rank") distributions and calculate the average sulfur rank depending on the sulfur concentration and temperature. This multi-scale approach allows us to bridge the gap between the local description of the covalent bonding process and the derivation of the macroscopic properties of the cathode. Our calculations show that the main reaction of the vulcanization process leads to high-probability states of sulfur chains cross-linking TBT units belonging to different polymer backbones, with a dominant rank around n = 5. In contrast, the connection of adjacent TBT units of the same polymer backbone by a sulfur chain is the side reaction. These results are experimentally supported by Raman spectroscopy.

Precise equilibrium structure determination of thiophene (c-C4H4S) by rotational spectroscopy-Structure of a five-membered heterocycle containing a third-row atom

The rotational spectrum of thiophene (c-C4H4S) has been collected between 8 and 360 GHz. Samples of varying deuterium-enrichment were synthesized to yield all possible deuterium-substituted isotopologues of thiophene. A total of 26 isotopologues have been measured and least-squares fit using A- and S-reduced distorted-rotor Hamiltonians in the Ir representation. The resultant rotational constants (A0, B0, and C0) from each reduction were converted to determinable constants (A″, B″, and C″) to remove the impact of centrifugal distortion. The computed vibrational and electron mass corrections [CCSD(T)/cc-pCVTZ] were applied to the determinable constants to obtain semi-experimental equilibrium rotational constants (Ae, Be, and Ce) for 24 isotopologues. A precise semi-experimental equilibrium (re SE) structure has been achieved from a least-squares fit of the equilibrium moments of inertia. The combination of the expanded isotopologue rotational data with high-level computational work establishes a precise re SE structure for this sulfur-containing heterocycle. The CCSD(T)/cc-pCV5Z structure has been obtained and corrected for the extrapolation to the complete basis set, electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born-Oppenheimer correction. The precise re SE structure is compared to the resulting "best theoretical estimate" structure. Several of the best theoretical re structural parameters fall within the narrow statistical limits (2σ) of the re SE results. The possible origin of the discrepancies for the computed parameters that fall outside the statistical uncertainties is discussed.

Exploring the effects of quantum decoherence on the excited-state dynamics of molecular systems

Fewest-switches surface hopping (FSSH) is employed in order to investigate the nonadiabatic excited-state dynamics of thiophene and related compounds and hence to establish a connection between the electronic system, the critical points in configuration space and the deactivation dynamics. The potential-energy surfaces of the studied molecules were calculated with complete active space self-consistent field and time-dependent density-functional theory. They are analyzed thoroughly to locate and optimize minimum-energy conical intersections, which are essential to the dynamics of the system. The influence of decoherence on the dynamics is examined by employing different decoherence schemes. We find that irrespective of the employed decoherence algorithm, the population dynamics of thiophene give results which are sound with the expectations grounded on the analysis of the potential-energy surface. A more detailed look at single trajectories as well as on the excited-state lifetimes, however, reveals a substantial dependence on how decoherence is accounted for. In order to connect these findings, we describe how ensemble averaging cures some of the overcoherence problems of uncorrected FSSH. Eventually, we identify carbon–sulfur bond cleavage as a common feature accompanying electronic transitions between different states in the simulations of all thiophene-related compounds studied in this work, which is of interest due to their relevance in organic photovoltaics.

Ground-state dissociation pathways for the molecular cation of 2-chlorothiophene: A time-of-flight mass spectrometry and computational study

RATIONALE

Halogenated thiophenes are an important class of compounds mostly used in the synthesis of various materials, showing unusual electronic and optical properties. The Thiophene Ring Fragmentation (TRF) process is widely used in synthetic chemistry. In this study, the fragmentation pattern of the molecular cation of halogenated thiophene, namely, 2-chlorothiophene, was monitored to establish its dissociation mechanism.

METHODS

The molecular cation of 2-chlorothiophene was prepared using multiphoton excitation using a laser at 235 nm. Various product ions upon fragmentation of the molecular ion were mass analyzed using time-of-flight mass spectrometry. Laser power dependence studies were also conducted for various product ions to arrive at the dissociation mechanism. Theoretical calculations were carried out to estimate the reaction enthalpies for various reactions and compared with the experimental data available in the literature.

RESULTS

The most abundant product ion was observed as the HCS⁺ radical cation followed by the C₃ H₃ ⁺ ion and the H₂ CCCCS⁺ radical cation. Other product ions such as SCCl⁺ , ClHCCS⁺ radical cations were also observed to a lesser extent in the fragmentation pattern of the parent molecular ion. Various dissociation channels were identified and supported with ab initio calculation. It has been inferred that the TRF process is usually initiated by the H/Cl atom transfer process. The appearance energies of the various fragment ions were also estimated theoretically and compared with literature values.

CONCLUSIONS

In conclusion, the fragmentation pattern of the molecular cation of 2-chlorothiophene was studied and the formation mechanisms of various product ions have been assigned. The appearance energies of the various fragment ions were also calculated. Finally, it is inferred that a TRF process is initiated by the H/Cl atom migration and subsequent ring opening either by C-C or C-S bond cleavage leading to the various isomers and their subsequent fragmentation. The ionization energies were accurately predicted for various species using ab initio calculation.

Exploring the Dynamics of the Photoinduced Ring-Opening of Heterocyclic Molecules

Excited states formed by electron promotion to an antibonding σ* orbital are now recognized as key to understanding the photofragmentation dynamics of a broad range of heteroatom containing small molecules: alcohols, thiols, amines, and many of their aromatic analogues. Such excited states may be populated by direct photoexcitation, or indirectly by nonadiabatic transfer of population from some other optically excited state (e.g., a ππ* state). This Perspective explores the extent to which the fast-growing literature pertaining to such (n/π)σ*-state mediated bond fissions can inform and enhance our mechanistic understanding of photoinduced ring-opening in heterocyclic molecules.

Molecular Photofragmentation Dynamics in the Gas and Condensed Phases

Exciting a molecule with an ultraviolet photon often leads to bond fission, but the final outcome of the bond cleavage is typically both molecule and phase dependent. The photodissociation of an isolated gas-phase molecule can be viewed as a closed system: Energy and momentum are conserved, and the fragmentation is irreversible. The same is not true in a solution-phase photodissociation process. Solvent interactions may dissipate some of the photoexcitation energy prior to bond fission and will dissipate any excess energy partitioned into the dissociation products. Products that have no analog in the corresponding gas-phase study may arise by, for example, geminate recombination. Here, we illustrate the extent to which dynamical insights from gas-phase studies can inform our understanding of the corresponding solution-phase photochemistry and how, in the specific case of photoinduced ring-opening reactions, solution-phase studies can in some cases reveal dynamical insights more clearly than the corresponding gas-phase study.

Visualizing Excited-State Dynamics of a Diaryl Thiophene: Femtosecond Stimulated Raman Scattering as a Probe of Conjugated Molecules

Conjugated organic polymers based on substituted thiophene units are versatile building blocks of many photoactive materials, such as photochromic molecular switches or solar energy conversion devices. Unraveling the different processes underlying their photochemistry, such as the evolution on different electronic states and multidimensional structural relaxation, is a challenge critical to defining their function. Using femtosecond stimulated Raman scattering (FSRS) supported by quantum chemical calculations, we visualize the reaction pathway upon photoexcitation of the model compound 2-methyl-5-phenylthiophene. Specifically, we find that the initial wavepacket dynamics of the reaction coordinates occurs within the first ≈1.5 ps, followed by a ≈10 ps thermalization. Subsequent slow opening of the thiophene ring through a cleavage of the carbon-sulfur bond triggers an intersystem crossing to the triplet excited state. Our work demonstrates how a detailed mapping of the excited-state dynamics can be obtained, combining simultaneous structural sensitivity and ultrafast temporal resolution of FSRS with the chemical information provided by time-dependent density functional theory calculations.

Deactivation pathways of thiophene and oligothiophenes: internal conversion versus intersystem crossing

Quantum chemical calculations reveal that excited thiophene decays via a low lying conical intersection seam. In oligothiophenes barriers inhibit this passage while deactivation pathways via intersystem crossing channels open.

New insights into the by-product fatigue mechanism of the photo-induced ring-opening in diarylethenes

The photochromic properties of diarylethenes, some of the most studied class of molecular switches, are known to be controlled by non-adiabatic decay at a conical intersection seam. Nevertheless, as their fatigue-reaction mechanism - leading to non-photochromic products - is yet to be understood, we investigate the photo-chemical formation of the so-called by-product isomer using three complementary computational methods (MMVB, CASSCF and CASPT2) on three model systems of increasing complexity. We show that for the ring-opening reaction a transition state on S1(2A) involving bond breaking of the penta-ring leads to a low energy S1(2A)/S0(1A) conical intersection seam, which lies above one of the transition states leading to the by-product isomer on the ground state. Therefore, radiationless decay and subsequent side-product formation can take place explaining the photo-degradation responsible for the by-product generation in diarylethene-type molecules. The effect of dynamic electron correlation and the possible role of inter-system crossing along the penta-ring opening coordinate are discussed as well.

Excited state proton transfer dynamics of thioacetamide in S2(ππ*) state: resonance Raman spectroscopic and quantum mechanical calculations study

The photophysics and photochemistry of thioacetamide (CH3CSNH2) after excitation to the S2 electronic state were investigated by using resonance Raman spectroscopy in conjunction with the complete active space self-consistent field (CASSCF) method and density functional theory (DFT) calculations. The A-band resonance Raman spectra in acetonitrile, methanol, and water were obtained at 299.1, 282.4, 266.0, 252.7, and 245.9 nm excitation wavelengths to probe the structural dynamics of thioacetamide in the S2 state. CASSCF calculations were done to determine the transition energies and structures of the lower-lying excited states, the conical intersection points CI(S2/S1) and CI(S1/S0), and intersystem crossing points. The structural dynamics of thioacetamide in the S2 state was revealed to be along eight Franck-Condon active vibrational modes ν15, ν11, ν14, ν10, ν8, ν12, ν18, and ν19, mostly in the CC/CS/CN stretches and the CNH8,9/CCH5,6,7/CCN/CCS in-plane bends as indicated by the corresponding normal mode descriptions. The S2 → S1 decay process via the S2/S1 conical intersection point as the major channel were excluded. The thione-thiol photoisomerization reaction mechanism of thioacetamide via the S2,FC → S'1,min excited state proton transfer (ESPT) reaction channel was proposed.

Excited state dynamics of thiophene and bithiophene: new insights into theoretically challenging systems

ADC(2) surface hopping study of the ultrafast deactivation mechanisms for thiophene and bithiophene.

Decay dynamics of α,β-carboxylic methyl esters (CH3OCOCH:CHR) in the lower-lying excited states--resonance Raman and complete active space self-consistent field calculation study

The photophysics of two α,β-carboxylic methyl esters after excitation to the light absorbing S2(ππ(*)) state were studied by using the resonance Raman spectroscopy and complete active space self-consistent field (CASSCF) method calculations. The vibrational spectra were assigned on the basis of the experimental measurements and the B3LYP/6-31G(d) computations, as well as the normal mode analysis. The A-band resonance Raman spectra of methyl 2,4-pentadienoate (M24PDA) and methyl trans cronoate (MTCA) were measured to probe the structural dynamics in Franck-Condon region. CASSCF calculations were done to obtain the minimal excitation energies and geometric structures of the lower-lying singlet and triplet excited states, and the curve-crossing points. It was revealed that the short-time structural dynamics of M24PDA was dominated by the Cα=Cβ-C4=C5 stretch coordinate, while that of MTCA was mostly along the Cα=Cβ and the C=O stretch motion. Comparison of the structural dynamics of M24PDA and MTCA with that of 3-methyl-3-pentene-2-one (3M3P2O) indicated that the structural dynamics of MTCA is similar to that of 3M3P2O but different than that of M24PDA in that the variation of the Raman intensity ratios for ν7/ν8, (ν7+ν8)/2ν8, (ν7+2ν8)/3ν8, (ν7+3ν8)/4ν8 of MTCA is similar to that of 3M3P2O but different from that of M24PDA. It is found that the substitution of methyl group in the α(')-position of α,β-enones by methoxyl group does not substantially affect the short-time structural dynamics, while the substitution of vinyl group in the β-position changes significantly the short-time structural dynamics and the subsequent decay processes. A detailed decay mechanism is proposed. Two sub-processes which consider the reconjugation and the subsequent charge-transfer reaction of O=C-Cα=Cβ chromophore were postulated to describe the variation of short-time structural dynamics with the different substitution.

Scanning angle Raman spectroscopy of poly(3-hexylthiophene)-based films on indium tin oxide, gold, and sapphire surfaces

Interest in realizing conjugated polymer-based films with controlled morphology for efficient electronic devices, including photovoltaics, requires a parallel effort to characterize these films. Scanning angle (SA) Raman spectroscopy is applied to measure poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM)-blend morphology on sapphire, gold, and indium tin oxide interfaces, including functional organic photovoltaic devices. Nonresonant SA Raman spectra are collected in seconds with signal-to-noise ratios that exceed 80, which is possible due to the reproducible SA signal enhancement. Raman spectra are collected as the incident angle of the 785 nm excitation laser is precisely varied upon a prism/sample interface from approximately 35 to 70°. The width of the ∼1447 cm(-1) thiophene C═C stretch is sensitive to P3HT order, and polymer order varied depending on the underlying substrate. This demonstrates the importance of performing the spectroscopic measurements on substrates and configurations used in the functioning devices, which is not a common practice. The experimental measurements are modeled with calculations of the interfacial mean square electric field to determine the distance dependence of the SA Raman signal. SA Raman spectroscopy is a versatile method applicable whenever the chemical composition, structure, and thickness of interfacial polymer layers need to be simultaneously measured.